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INTRODUCTION
Voice
Analog voice date is encoded into 64K-bps pulse-code modulation (PCM)
samples using an encoding technique known as the Mu-255 law. Details of this
encoding technique are not given here. The important point is that each DS1
channel can transport PCM-encoded 64K-bps voice signals.
Voice-grade
data
Digital data
Voice grade data is also called PCM Data and voiceband analog data. Modems
receive digital data, convert the data to an analog voiceband signal, and transmit
it over analog phone lines. Whenever the modem connects to a digital switch,
the modem analog output signal undergoes the same PCM encoding process as
voice. Therefore, the modem output is termed voice-grade data.
This two-step process of first converting digital data to analog data and then to
64K-bps PCM data is necessary for transmitting data on DS1/DMI facilities that
are either administered for robbed-bit signaling (RBS) or routed over a
combination of digital and analog sections.
An attribute of voice-grade data is that signaling information can be inserted into
the least-significant bit (LSB) of the PCM words without destroying the data.
This capability cannot be done for those DS1 facilities that transmit digital data
(described below).
Voice-grade data calls placed over DS1/DMI facilities, which use RBS, require
the use of a modem to permit this two-step conversion. Actually, the modem
pool (modem-to-switch) interface does this conversion.
NOTE: Voice-grade data is limited to speeds provided by the modem
(typically 19.2K-bps or less). However, DS1 channels accommodate data at
rates up through 56K-bps.
Digital data operates at 64K-bps and 56K-bps rates. Computers and data
terminals generate digital data. The computer ports and data terminals interface
to data modules. Data modules transmit the digital data (in digital form) to the
switch. When this digital data is switched into a DS1/DMI channel without any
intervening processes (such as modem pool conversion or embedded signaling
information), the channel is said to provide 64K-bps data capability (also known
as mode-1 data). The important point here is that when a DS1/DMI signal
consists of digital data, every bit that goes in at one end must come out the other
end unaltered; otherwise, the data would be destroyed.
Data modules support 56K-bps digital data over robbed-bit facilities.
NOTE: Although the digital data channels transmit synchronous 64K-bps
data, computer ports and data terminals do not typically generate digital data
at this rate. Data modules provide data rate adaptation (modes 0, 1, and 2)
and generate nulls or fill characters (as required) for maintaining the 64K-
bps data rate.